Torque detection device and electric power steering system

ABSTRACT

A torque detection device includes: a torsion bar that couples a first shaft to a second shaft; a magnet that is fixed to the first shaft; and a pair of magnetic yokes that are fixed to the second shaft and that are arranged to face each other in an axial direction. Each of the magnetic yokes includes a yoke ring and a plurality of lugs that are arranged in a circumferential direction on the corresponding yoke ring. Each yoke ring includes an extending portion that extends radially outward from base portions of the lugs, and a bent portion that is bent in the axial direction from a radially outer end portion of the extending portion. The outer size of the pair of magnetic yokes in the axial direction is larger than or equal to the length of the magnet in the axial direction.

INCORPORATION BY REFERENCE/RELATED APPLICATION

This application claims priority to Japanese Patent Application No.2011-244583 filed on Nov. 8, 2011 the disclosure of which, including thespecification, drawings and abstract, is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a torque detection device that detects, forexample, a steering torque of a steering wheel and an electric powersteering system that includes the torque detection device.

2. Discussion of Background

A torque sensor described in US 2006/0137474 A is formed of a torsionbar, a ring-shaped magnet, a pair of annular magnetic yokes, a pair ofmagnetic flux concentration rings, a magnetic sensor, and the like. Thetorsion bar couples an input shaft to an output shaft such that theinput shaft and the output shaft are coaxial with each other. The magnetis connected to an end portion of the input shaft. The magnetic yokesare connected to an end portion of the output shaft. The magnetic fluxconcentration rings are arranged in proximity to the outer peripheriesof the magnetic yokes. Each of the magnetic yokes has lugs all around.The number of the lugs is equal to the number of N-poles and S-poles ofthe magnet. The magnetic flux concentration rings concentrate magneticfluxes induced from the magnet by the magnetic yokes. The magneticsensor is placed between the magnetic flux concentration rings, anddetects the magnetic flux density generated due to the magnetic fluxesconcentrated by the magnetic flux concentration rings.

When torque (steering torque) is input into a portion between the inputshaft and the output shaft through a steering operation of a steeringwheel, the torsion bar is twisted, and the relative position between themagnet and the magnetic yokes in the circumferential direction changes.The torque sensor detects the steering torque input into the portionbetween the input shaft and the output shaft on the basis of themagnetic flux density that changes in the magnetic yokes in accordancewith the change in the relative position.

In the torque sensor described in US 2006/0137474 A, a portion of eachmagnetic yoke, at which each lug is provided, has an L-shape such that,when viewed from the circumferential direction of the magnetic yoke, theportion extends radially inward of the magnetic yoke, bends at asubstantially right angle and then extends in the axial direction of themagnetic yoke (see FIG. 5 of US 2006/0137474 A). The distal end portionsof the lugs of the respective magnetic yokes are arranged alternately inthe circumferential direction. In the torque sensor, the lugs throughwhich magnetic fluxes flow are desired to be elongated by extending thelugs in the axial direction in order to improve torque detectioncapability and torque detection accuracy. However, if the size of eachlug in the axial direction (hereinafter, in the specification, referredto as “effective length” where appropriate) is elongated excessively,the distal end of the lug excessively approaches the opposed magneticyoke. Thus, the magnetic yokes may interfere with each other portionsclose to each other. In addition, the magnetic flux is likely tounexpectedly leak from the magnetic yoke at that portion of each lug.Therefore, it is difficult for the magnetic sensor to highly accuratelydetect the magnetic flux density in the magnetic yokes (the density ofmagnetic fluxes concentrated by the magnetic flux concentration rings).That is, it is difficult for the torque sensor to highly accuratelydetect a torque.

In addition, it is always desired to reduce the size of the torquesensor.

SUMMARY OF THE INVENTION

The invention provides a torque detection device with improved detectioncapability and detection accuracy while preventing interference betweena pair of magnetic yokes, and an electric power steering system thatincludes the torque detection device. In addition, the inventionprovides a more compact torque detection device, and an electric powersteering system that includes the torque detection device.

According to a feature of an example of the invention, each yoke ringhas an extending portion that extends radially outward from baseportions of lugs and a bent portion that is bent in an axial directionfrom a radially outer end portion of the extending portion, distal endportions of the lugs of one of the magnetic yokes and distal endportions of the lugs of the other magnetic yoke are arranged alternatelyin a circumferential direction, and the outer size of the pair ofmagnetic yokes in the axial direction is larger than or equal to thelength of a magnet in the axial direction.

In each of the magnetic yokes, the effective length of each lug isincreased by the length of the bent portion. In this way, the torquedetection device is provided with improved torque detection capabilityand torque detection accuracy. Further, the outer size of the magneticyokes in the axial direction is larger than or equal to the length ofthe magnet in the axial direction. Therefore, it is possible to set thelength of the magnet to a value equal to or smaller than the outer size(overall length) of the magnetic yokes. Accordingly, it is possible toreduce the size of the torque detection device.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of theinvention will become apparent from the following description of exampleembodiment with reference to the accompanying drawings, wherein likenumerals are used to represent like elements and wherein:

FIG. 1 is a schematic view that shows the schematic configuration of anelectric power steering system that includes a torque detection deviceaccording to an embodiment of the invention;

FIG. 2 is an exploded perspective view of the torque detection device;

FIG. 3 is a sectional view of the torque detection device;

FIG. 4 is a side view of a pair of magnetic yokes as viewed from aposition radially outward of the magnetic yokes;

FIG. 5 is a perspective view of a magnetic yoke unit;

FIG. 6A is a schematic view that shows a state where a torsion bar istwisted in one direction from a steering neutral state in the torquedetection device;

FIG. 6B is a schematic view that shows the steering neutral state;

FIG. 6C is a schematic view that shows a state where the torsion bar 16is twisted in the direction opposite to the direction in FIG. 6A fromthe steering neutral state in the torque detection device;

FIG. 7A is an axial sectional view of one of the magnetic yokes;

FIG. 7B is a sectional view of a magnetic yoke according to related art;

FIG. 8A is a schematic view of a magnet and a pair of magnetic yokes ina torque detection device according to the related art as viewed from aposition radially outward of the magnet and the magnetic yokes;

FIG. 8B is a schematic view of a magnet and the magnetic yokes in thetorque detection device according to the invention as viewed from aposition radially outward of the magnet and the magnetic yokes;

FIG. 9 is a view in which a lug of the torque detection device accordingto the invention is superimposed on a lug of the torque detection deviceaccording to the related art;

FIG. 10 is a graph that illustrates a state where the area of an overlapbetween a lug and one pole of the magnet changes as the torsion bar istwisted in the torque detection device; and

FIG. 11 is a side view of each magnetic yoke according to anotherembodiment of the invention as viewed from a position on a flat planethat passes through a circular central position of the magnetic yoke andextends in an axial direction of the magnetic yoke.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the invention will be described withreference to the accompanying drawings.

FIG. 1 is a schematic view that shows the schematic configuration of anelectric power steering system 1 that includes a torque detection device17 according to an embodiment of the invention. The electric powersteering system 1 includes a steering shaft 3, an intermediate shaft 5,a pinion shaft 7 and a rack shaft 10. The steering shaft 3 is coupled toa steering wheel 2. The intermediate shaft 5 is coupled to the steeringshaft 3 via a universal joint 4. The pinion shaft 7 is coupled to theintermediate shaft 5 via a universal joint 6. The rack shaft 10 has arack 9 that is in mesh with a pinion 8 provided at the distal endportion of the pinion shaft 7, and extends in the lateral direction of avehicle.

The rack shaft 10 is supported by a tubular housing 11 so as to bemovable in the axial direction. Tie rods 12 are coupled to respectiveend portions of the rack shaft 10. Each tie rod 12 is coupled to acorresponding one of steered wheels 13 via a corresponding one ofknuckle arms (not shown). When the steering wheel 2 is operated torotate the steering shaft 3, the rotation is transmitted to the pinion 8via, for example, the intermediate shaft 5, and is converted into alinear motion of the rack shaft 10 in the lateral direction of thevehicle by the pinion 8 and the rack 9. In this way, the steered wheels13 are steered.

The steering shaft 3 includes a first steering shaft 14 and a secondsteering shaft 15. The first steering shaft 14 may function as a firstshaft coupled to the steering wheel 2. The second steering shaft 15 mayfunction as a second shaft coupled to the universal joint 4. The firstand second steering shafts 14, 15 are coaxially coupled to each othervia a torsion bar 16 that may function as an elastic member. The firstand second steering shafts 14, 15 are able to transmit torque to eachother, and are rotatable relative to each other within a predeterminedrange. When torsional torque that corresponds to the steering torque ofthe steering wheel 2 is input into a portion between the first andsecond steering shafts 14, 15 through a steering operation of thesteering wheel 2, the torsion bar 16 is twisted. At this time, the firstand second steering shafts 14, 15 are rotated relative to each other.

The electric power steering system 1 includes the torque detectiondevice 17, a vehicle speed sensor 18, a steering assist electric motor19 and an ECU 20. The torque detection device 17 detects a steeringtorque applied to the steering wheel 2. The vehicle speed sensor 18detects a vehicle speed. The ECU 20 serves as a control unit andincludes a microcomputer that executes drive control of the electricmotor 19 on the basis of the detected vehicle speed and the detectedsteering torque.

The torque detection device 17 detects a steering torque applied to thefirst and second steering shafts 14, 15 from a change in magnetic fluxdensity based on a relative rotational displacement between the firststeering shaft 14 and the second steering shaft 15 due to a torsion ofthe torsion bar 16. When the ECU 20 drives the steering assist electricmotor 19, the rotation (driving force) output from the electric motor 19is reduced in speed by a speed reduction mechanism 21, such as a wormgear mechanism, and is then transmitted to the second steering shaft 15.The torque transmitted to the second steering shaft 15 is furthertransmitted to a steered mechanism 22 via, for example, the intermediateshaft 5. Thus, driver's steering operation is assisted by the drivingforce of the electric motor 19. The steered mechanism 22 includes thepinion shaft 7, the rack shaft 10, the tie rods 12, the knuckle arms,and the like.

FIG. 2 is an exploded perspective view of the torque detection device17. FIG. 3 is a sectional view of the torque detection device 17. Asshown in FIG. 2 and FIG. 3, one end 16A of the torsion bar 16 isconnected to the first steering shaft 14 by a pin (not shown), or thelike, and the other end 16B of the torsion bar 16 is connected to thesecond steering shaft 15 by a pin (not shown), or the like.

The torque detection device 17 includes the above-described torsion bar16, a magnet 25, a pair of magnetic yokes 26, a pair of magnetic fluxconcentration rings 28 and a pair of Hall ICs 30. The magnetic yokes 26are made of a soft magnetic material. The magnetic flux concentrationrings 28 induce magnetic fluxes from the magnetic yokes 26. The Hall ICs30 may function as a magnetic sensor. In the following description, whenone of the magnetic yokes 26, one of the magnetic flux concentrationrings 28 and one of the Hall ICs 30 are indicated, “A” is suffixed onthe reference numerals, and “B” is suffixed on the reference numeralswhen the other ones are indicated. In FIG. 2, the upper magnetic yoke 26is defined as the magnetic yoke 26A, and the lower magnetic yoke 26 isdefined as the magnetic yoke 26B. The upper magnetic flux concentrationring 28 is defined as the magnetic flux concentration ring 28A, and thelower magnetic flux concentration ring 28 is defined as the magneticflux concentration ring 28B. In addition, the left Hall IC 30 is definedas the Hall IC 30A, and the right Hall IC 30 is defined as the Hall IC30B. Note that the up-down directions in FIG. 2 and FIG. 3 coincide witheach other.

The magnet 25 has an annular shape (more specifically, cylindricalshape), and is fixed to one end of the first steering shaft 14 so as tobe rotatable together with each other. Multiple N-poles and multipleS-poles are alternately formed in the circumferential direction of themagnet 25. For example, a ferrite magnet may be used as the magnet 25.Because the magnet 25 is coaxially fixed to the first steering shaft 14,the axis of the magnet 25 and the axis of the first steering shaft 14coincide with each other.

The magnetic yokes 26 are fixed to one end of the second steering shaft15 so as to be rotatable around the magnet 25. Each of the magneticyokes 26 has an annular shape. More specifically, each of the magneticyokes 26 has an annular yoke ring 40 and a plurality of lugs 41 formedintegrally with the annular yoke ring 40. The yoke rings 40 are spacedapart from each other and face each other. The lugs 41 are provided at aplurality of circumferential positions one by one on the inner peripheryof each yoke ring 40.

Each yoke ring 40 includes rectangular thin-sheet-shaped extendingportions 40A and an annular bent portion 40B. The extending portions 40Aeach extend radially outward from a base portion 41A of each lug 41. Thebent portion 40B forms the contour of the yoke ring 40. Each extendingportion 40A is thin in the axial direction of the yoke ring 40. All theextending portions 40A are arranged at equal intervals in thecircumferential direction. Accordingly, the lugs 41 are arranged atequal intervals in the circumferential direction while protruding(extending) in the axial direction from the extending portions 40A. Eachlug 41 has a thin-plate shape in the radial direction of the yoke ring40. In addition, the bent portion 40B has a certain width in the axialdirection, and has a small thickness in the radial direction. Theextending portions 40A are connected to one edge (the upper edge in themagnetic yoke 26A, and the lower edge in the magnetic yoke 26B) in theaxial direction of the bent portion 40B.

As shown in FIG. 3, when the yoke ring 40 (left-side portion in FIG. 3)of the upper magnetic yoke 26A is viewed from the circumferentialdirection, the bent portion 408 is bent in the axial direction (downwarddirection in which the lugs 41 extend) from the radially outer endportions of the extending portions 40A in a state where the lugs 41extend downward. The extending portions 40A respectively extend radiallyoutward from the base portions 41 A of the lugs 41. In addition, asshown in FIG. 3, when the yoke ring 40 (right-side portion in FIG. 3) ofthe lower magnetic yoke 26B is viewed in the circumferential direction,the bent portion 40B is bent in the axial direction (upward direction inwhich the lugs 41 extend) from the radially outer end portions of theextending portions 40A in a state where the lugs 41 extend upward. Theextending portions 40A extend radially outward from the base portions 41A of the lugs 41. Therefore, a portion of the magnetic yoke 26A at whicheach lug 41 is provided has a J-shape (when the bent portion 40B isshorter than each lug 41 in the axial direction) or a U-shape (when thebent portion 40B is equal in length in the axial direction to each lug41) in cross section.

FIG. 4 is a side view when the magnetic yokes 26 are viewed from aposition radially outward of the magnetic yokes 26. As shown in FIG. 4,when the magnetic yokes 26 are viewed from a position radially outwardof the magnetic yokes 26, the lugs 41 in each of the magnetic yokes 26each have the same shape and the same size. More specifically, when, ineach lug 41, a portion located at a farthest end from the base portion41 A and at the center in the width direction (circumferential directionof the magnetic yoke 26) is defined as a distal end 41B, each lug 41 hasa curved profile R of which the width becomes narrower toward the distalend 41B and of which the corners rounded, as viewed from the radialdirection.

In FIG. 4, the profile R has a parabolic shape (conic curve) and has asubstantially U-shape. When each lug 41 has the parabolic profile R, theprofile R is easily reproducible, so it is possible to relatively easilyform each lug 41. Alternatively, the profile R may be formed byconnecting a plurality of curves having different curvature radii. Inthis case, by forming the profile R of each lug 41 from a plurality ofcurves, it is possible to form the lug 41 having a complex curve-shapedprofile R according to a design request.

FIG. 5 is a perspective view of a magnetic yoke unit 33. With regard tothe magnetic yokes 26, the torque detection device 17 includes acylindrical synthetic resin member 32 as shown in FIG. 5. The magneticyokes 26 are coaxially arranged to face each other with a predeterminedgap left therebetween in the axial direction, and are molded in thesynthetic resin member 32 in a state where the magnetic yokes 26 arepositioned such that the distal end portions (distal end 41B-sideportions) of the lugs 41 are alternately arranged in the circumferentialdirection (also see FIG. 4). In the magnetic yokes 26, the innerperipheries of the lugs 41 are exposed at an inner periphery 32A of thesynthetic resin member 32 in a state where the inner peripheries of thelugs 41 are substantially flush with the inner periphery 32A. In themagnetic yokes 26, the outer peripheries of the bent portions 40B of theyoke rings 40 are exposed at an outer periphery 32B of the syntheticresin member 32 in a state where the outer peripheries of the bentportions 40B of the yoke rings 40 are substantially flush with the outerperiphery 32B.

Hereinafter, a set of the paired magnetic yokes 26 and the syntheticresin member 32 that holds the magnetic yokes 26 is referred to as themagnetic yoke unit 33. The magnetic yoke unit 33 has a cylindricalshape. As shown in FIG. 3, in the completed torque detection device 17,the magnetic yoke unit 33 (in other words, the magnetic yokes 26) iscoaxially fixed to the second steering shaft 15 and are also coaxialwith the first steering shaft 14, and surrounds the magnet 25 from theradially outer side in a noncontact state. Therefore, the magnetic yokeunit 33 is coaxial with the magnet 25. Note that the axial directions ofthe magnetic yokes 26 (magnetic yoke unit 33), the first steering shaft14, the second steering shaft 15 and the magnet 25 coincide with oneanother, and are collectively referred to as “axial direction X1”.

Then, in the magnetic yokes 26 positioned in the magnetic yoke unit 33,the outer size Q of the magnetic yokes 26 in the axial direction X1,that is, a value Q that is obtained by adding the thicknesses T of therespective extending portions 40A to a clearance K between the extendingportions 40A of the respective yoke rings 40 (=K+T+T) is larger than orequal to the length G of the magnet. Therefore, in the axial directionX1, the magnet 25 is located inside the magnetic yokes 26.

As shown in FIG. 2, the magnetic flux concentration rings 28 are annularmembers made of a soft magnetic material. The magnetic fluxconcentration rings 28 are arranged so as to be relatively rotatablearound the magnetic yokes 26 while surrounding the magnetic yokes 26from the radially outer side. The magnetic flux concentration rings 28are respectively magnetically coupled to the magnetic yokes 26.Specifically, the upper magnetic flux concentration ring 28A faces thebent portion 40B of the upper magnetic yoke 26A from the radially outerside over all around in a noncontact state, and the lower magnetic fluxconcentration ring 28B faces the bent portion 40B of the lower magneticyoke 26B from the radially outer side over all around in a noncontactstate (see FIG. 3). At this time, the magnetic yokes 26 and the magneticflux concentration rings 28 are coaxial with each other.

The magnetic flux concentration rings 28 respectively have sheet-shapedmagnetic flux concentration plates 35, 36 at one portions in thecircumferential direction. The magnetic flux concentration plates 35, 36are spaced apart from each other and face each other in the axialdirection X1. The magnetic flux concentration rings 28 are able to guidemagnetic fluxes generated in the magnetic yokes 26 to the correspondingmagnetic flux concentration plates 35, 36 and concentrate the magneticfluxes between the magnetic flux concentration plates 35, 36. The HallICs 30 are placed in an air gap 37 formed between the magnetic fluxconcentration plates 35, 36 (also see FIG. 3). The function of the HallICs 30 will be described later.

In the thus configured torque detection device 17, as the magnet 25 andthe magnetic yokes 26 rotate relative to each other due to a torsion ofthe torsion bar 16 as a result of a steering operation of the steeringwheel 2, the magnetic flux density between the magnetic yokes 26changes.

FIG. 6A to FIG. 6C are schematic views for illustrating the operation ofthe torque detection device 17. FIG. 6A shows a state where the torsionbar 16 is twisted in one direction from a steering neutral state. FIG.6B shows the steering neutral state. FIG. 6C shows a state where thetorsion bar 16 is twisted in the direction opposite to the direction inFIG. 6A from the steering neutral state.

FIG. 6B shows a state where the steering wheel 2 is not operated(steering neutral state). In this case, no steering torque is applied tothe first and second steering shafts 14, 15, and, in the magnetic yokes26, the distal ends 41B of the respective lugs 41 are arranged so as topoint at boundaries between the N-poles of the magnet 25 (portionsshaded by dots in the magnet 25 shown in FIG. 6A to FIG. 6C) and theS-poles of the magnet 25 (portions with no dots in the magnet 25 shownin FIG. 6A to FIG. 6C). In this state, because the same number of linesof magnetic force from the N-poles and S-poles of the magnet 25 enterinto and exit from the lugs 41 of the magnetic yokes 26, the lines ofmagnetic force are closed in each of the magnetic yoke 26A and themagnetic yoke 26B. Thus, no magnetic flux leaks to a portion between themagnetic yoke 26A and the magnetic yoke 26B, and the magnetic fluxdensity between the magnetic yoke 26A and the magnetic yoke 26B areunchanged at zero.

On the other hand, when a torsional torque is input into a portionbetween the first steering shaft 14 and the second steering shaft 15through a steering operation of the steering wheel 2 and thus thetorsion bar 16 (see FIG. 3) is twisted, the relative position betweenthe magnet 25 fixed to the first steering shaft 14 and the magneticyokes 26 fixed to the second steering shaft 15 changes in thecircumferential direction. Thus, as shown in FIG. 6A and FIG. 6C, thedistal ends 41B of the lugs 41 of the magnetic yokes 26 no longercoincide with the boundaries between the N-poles and S-poles of themagnet 25. Therefore, lines of magnetic force having an N or S polarityincrease in each of the magnetic yokes 26. At this time, because linesof magnetic force having opposite polarities increase in the respectivemagnetic yoke 26A and magnetic yoke 26B, the magnetic flux density(change in magnetic flux density) is generated between the magnetic yoke26A and the magnetic yoke 26B. The magnetic flux density issubstantially proportional to the torsion amount of the torsion bar 16,and the polarity is inverted on the basis of the torsional direction ofthe torsion bar 16.

As shown in FIG. 3, when the magnetic flux density is generated betweenthe magnetic yoke 26A and the magnetic yoke 26B as described above, themagnetic flux concentration rings 28 guide the magnetic fluxes generatedin the magnetic yokes 26 to the magnetic flux concentration plates 35,36 and concentrate the magnetic fluxes between the magnetic fluxconcentration plates 35, 36. Therefore, the magnetic flux density isalso generated in the air gap 37 formed between the magnetic fluxconcentration plates 35, 36, as well as between the magnetic yoke 26Aand the magnetic yoke 26B. The Hall ICs 30 placed in the air gap 37detect the magnetic flux density generated due to the magnetic fluxes(originally, magnetic fluxes generated in the magnetic yokes 26)concentrated in the air gap 37 by the magnetic flux concentration rings28, and acquire the magnetic flux density as electric signals.

The ECU 20 (see FIG. 1) calculates the torsion amount of the torsion bar16, that is, the steering torque input into the steering shaft 3, on thebasis of the electric signals from the Hall ICs 30.

FIG. 7A is a side view of one of the magnetic yokes 26 when viewed froma position on a flat plane that passes through the circular centralposition and extends in the axial direction. In FIG. 7A, part of themagnetic yoke 26 is shown in cross section. FIG. 7B is a sectional viewof a magnetic yoke 26 in related art.

As shown in FIG. 7A, in the present embodiment of the invention, in eachof the magnetic yokes 26, the effective length Y (maximum length in theaxial direction X1) of each lug 41 is increased by the length of thebent portion 40B. More specifically, at a portion of each magnetic yoke26, at which the lug 41 is provided, the bent portion 40B is spacedapart from the bent portion 40B of the opposed magnetic yoke 26 (themagnetic yoke 26 that should be located above the magnetic yoke 26 shownin

FIG. 7A), the extending portion 40A extends from the bent portion 40B tothe lug 41, and the lug 41 extends toward the opposed magnetic yoke 26.Therefore, in comparison with the case where no bent portion 40B isprovided as in the related art shown in FIG. 7B, the effective length Yof each lug 41 is increased by the length of the bent portion 40B. Inthis way, larger magnetic fluxes are acquired in each of the lugs 41.Therefore, the torque detection device 17 has improved torque detectioncapability and torque detection accuracy.

In addition, as shown in FIG. 3, when the bent portions 40B areprovided, a portion of each magnetic yoke 26, which radially faces themagnetic flux concentration ring 28, serves as the bent portion 40B, andit is possible to ensure a larger area in which the magnetic yokes 26and the magnetic flux concentration rings 28 face each other. Therefore,magnetic fluxes are efficiently passed from the magnetic yokes 26 to themagnetic flux concentration rings 28. With this configuration as well,the torque detection device 17 is provide with improved torque detectioncapability and torque detection accuracy.

Here, in each lug 41 having the large effective length Y (see FIG. 7A),the base (base portion 41A) that extends to the bent portion 40B via theextending portion 40A is enlarged, and the distal end 41B-side portionis not brought close to the opposed magnetic yoke 26. Therefore, themagnetic yokes 26 do not interfere with each other, and magnetic fluxesdo not easily leak between the magnetic yokes 26.

As described above, it is possible to improve the detection capabilityand detection accuracy while preventing interference between themagnetic yokes 26. In addition, because the outer size Q of the magneticyokes 26 in the axial direction X1 is larger than or equal to the lengthG of the magnet 25, it is possible to set the length of the magnet 25equal to or smaller than the outer size Q (overall length) of themagnetic yokes 26. Accordingly, it is possible to reduce the size of thetorque detection device 17.

FIG. 8A is a schematic view of the magnet 25 and a pair of the magneticyokes 26 in the torque detection device 17 according to the related artas viewed from a position radially outward of the magnet 25 and themagnetic yokes 26. FIG. 8A shows part of the magnet 25 and part themagnetic yokes 26 in the circumferential direction. FIG. 8B is aschematic view of the magnet 25 and a pair of the magnetic yokes 26 inthe torque detection device 17 according to the present embodiment asviewed from a position radially outward of the magnet 25 and themagnetic yokes 26. FIG. 8B shows part of the magnet 25 and part themagnetic yokes 26 in the circumferential direction. FIG. 9 is a view inwhich the lug 41 of the torque detection device 17 according to thepresent embodiment of the invention is superimposed on the lug 41 of thetorque detection device 17 according to the related art.

The tapered isosceles trapezoidal lugs 41 (see FIG. 8A) in the relatedart are compared with the lugs 41 (see FIG. 8B) according to the presentembodiment of the invention. These lugs 41 are equal in length in theaxial direction X1 and equal in maximum width W in the circumferentialdirection of the base portion 41 A (the circumferential direction of themagnetic yoke 26) (see FIG. 9).

In the present embodiment of the invention, the width of each lug 41 ofeach magnetic yoke 26 becomes narrower toward its distal end and has thecurved profile R with rounded corners when viewed from the radialdirection of the magnetic yoke 26. Therefore, each lug 41 has aconvex-curved shape toward the distal end 41B. Therefore, each lug 41according to the present embodiment of the invention does not have twocorners C that are present at the distal end 41 B side of thetrapezoidal lug 41 according to the related art. Therefore, in thepresent embodiment of the invention, a clearance S (a portion surroundedby dashed lines) between the distal end 41B side of each lug 41 and theyoke ring 40 of the opposed magnetic yoke 26 is larger than that in therelated art by an amount corresponding to the two corners C.

Because the lug 41 having a convex-curved shape in the presentembodiment of the invention has no sharp edges, the magnetic fluxes ofthe magnetic yokes 26 are less likely to leak. Furthermore, if the lugs41 having a convex-curved shape are employed, the clearance S betweenthe distal end 41B side of the lug 41 and the yoke ring 40 of theopposed magnetic yoke 26 is relatively large. Therefore, with thisconfiguration as well, magnetic fluxes are less likely to leak alsobetween each lug 41 and the opposed magnetic yoke 26. Therefore, theHall ICs 30 (see FIG. 3) are able to highly accurately detect themagnetic flux density in the magnetic yokes 26 via the magnetic fluxconcentration rings 28. As a result, the torque detection device 17 isprovided with improved detection capability and detection accuracy.

FIG. 10 is a graph that illustrates the state where the area of anoverlap between each lug 41 and one pole of the magnet 25 changes as thetorsion bar 16 is twisted in the torque detection device 17. Inaddition, in the case of the lugs 41 according to the present embodimentof the invention, which have such a convex-curved shape, when thetorsion bar 16 is twisted from the above-described steering neutralstate (see FIG. 6B and FIG. 8B), the area of an overlap (when viewed inthe radial direction) between the N-poles or S-poles of the magnet 25and the lugs 41 changes on the basis of the torsion amount of thetorsion bar 16 (in other words, the amount of relative rotation betweenthe magnet 25 and the magnetic yokes 26) as shown in FIG. 10.

In the present embodiment, because each of the lugs 41 has aconvex-curved shape, regions (see hatched portions in FIG. 9) from thebase portion 41A to a portion before the distal end 41B are wider thanthat of each of the lugs 41 according to the related art. Therefore, inthe case of the lugs 41 according to the present embodiment of theinvention, the area of an overlap between the N-poles or S-poles and thelugs 41 changes in a state where the area is larger that in the case ofthe lugs 41 according to the related art. In addition, in the case ofthe lugs 41 according to the present embodiment of the invention, thearea changes more linearly and the rate of the change in the area ishigher than in the case of the lugs 41 according to the related art(particularly, regions near the start of rising and near the end ofrising in the curve of the graph). Therefore, in the torque detectiondevice 17 that uses the lugs 41 according to the present embodiment ofthe invention, the efficiency of detecting magnetic fluxes (in otherwords, torque) also improves.

In addition, as shown in FIG. 4, in each of the magnetic yokes 26, themaximum circumferential width W of the base portion 41A of each lug 41is preferably smaller than or equal to half the pitch P of the adjacentlugs 41 in the circumferential direction. In this case, it is possibleto ensure a sufficient clearance between adjacent lugs 41 among the lugs41 of the two magnetic yokes 26, which are arranged alternately in thecircumferential direction. Therefore, it is possible to prevent aleakage of magnetic fluxes between adjacent lugs 41. Thus, it ispossible to further improve the detection capability and detectionaccuracy.

In addition, as shown in FIG. 3, in the torque detection device 17, theHall ICs 30 detect the density of magnetic fluxes generated in themagnetic yokes 26 in a state where the magnetic fluxes are averaged bythe magnetic flux concentration rings 28. Therefore, it is possible todetect the magnetic flux density in the magnetic yokes 26 withoutvariations. As a result, it is possible to further improve the detectioncapability and detection accuracy. The detection capability anddetection accuracy of the steering torque of the steering wheel 2 in thetorque detection device 17 are improved in this way. Therefore, in theelectric power steering system 1, it is possible to highly accuratelyassist the steering operation of the steering wheel 2 with the use ofthe electric motor 19.

The invention is not limited to the above-described embodiment. Forexample, in the above-described embodiment, the two Hall ICs 30 are usedas the magnetic sensor (see FIG. 2). Alternatively, a single Hall IC maybe used. In addition, a magnetoresistive element (MR element) may beused as the magnetic sensor, instead of the Hall IC. In addition, thenumber of poles of the magnet 25 may be selectively set to, for example,12 poles, 16 poles, 18 poles, or 24 poles.

FIG. 11 is a side view of each magnetic yoke 26 according to anotherembodiment when viewed from a position on a flat plane that passesthrough the circular central position of the magnetic yoke 26 andextends in the axial direction. In the yoke ring 40 of each magneticyoke 26 according to the above-described embodiment, the bent portion40B is bent in a direction, in which the lugs 41 extend, from theradially outer end portion of the extending portions 40A in the axialdirection X1 (see FIG. 7A). Alternatively, as shown in FIG. 11, the bentportion 40B may be bent in the direction opposite to the direction inwhich the lugs 41 extend. In this case, a portion of each magnetic yoke26 at which each lug 41 is provided has a crank shape when viewed fromthe circumferential direction.

Note that, as shown in FIG. 3, the clearance in the axial direction X1between the magnetic flux concentration rings 38 may be constant, andthe magnetic flux concentration rings 38 may be a magnetic fluxconcentration ring assembly 50 of which the relative position is set. Inthis case, when a clearance K in the axial direction X1 between theextending portions 40A of the magnetic yokes 26 is larger than a maximumlength P of the magnetic flux concentration ring assembly 50, the bentportion 40B may be bent in the direction in which the lugs 41 extend andthe bent portion 40B may face the corresponding magnetic fluxconcentration ring 38 from the radially inner side. On the other hand,when the clearance K is smaller than the maximum length P of themagnetic flux concentration ring assembly 50, the bent portion 40B maybe bent in the direction opposite to the direction in which the lugs 41extend (see FIG. 11) and the bent portion 40B may face the correspondingmagnetic flux concentration ring 38 from the radially inner side. Bychanging the bent direction of the bent portion 40B, it is possible touse the same magnetic flux concentration ring assembly 50.

In the above-described embodiment, the torque detection device thatincludes the magnetic flux concentration rings is applied to theelectric power steering system for an automobile. Alternatively, thetorque detection device according to the invention may be applied tosystems or devices other than the electric power steering system.

What is claimed is:
 1. A torque detection device, comprising: an elasticmember that couples a first shaft to a second shaft such that the firstshaft and the second shaft are rotatable relative to each other, andthat is twisted as a torsional torque is input into a portion betweenthe first shaft and the second shaft; an annular magnet that iscoaxially fixed to the first shaft, and in which N-poles and S-poles arealternately formed in a circumferential direction; a pair of annularmagnetic yokes that are coaxially fixed to the second shaft, thatsurround the magnet in a noncontact state, and that are arranged so asto face each other in an axial direction; a pair of annular magneticflux concentration rings that respectively surround the pair of magneticyokes and that guide and concentrate the magnetic fluxes generated inthe pair of magnetic yokes, and a magnetic sensor that is arrangedbetween the pair of magnetic flux concentration rings and that detectsthe density of magnetic fluxes concentrated by the magnetic fluxconcentration rings, wherein each of the pair of magnetic yokes includesan annular yoke ring and a plurality of lugs that are extended in theaxial direction from the corresponding yoke ring and that are arrangedat equal intervals in the circumferential direction, each yoke ringincludes an extending portion that extends radially outward from baseportions of the lugs and a bent portion that is bent in the axialdirection from a radially outer end portion of the extending portion,and distal end portions of the lugs of one of the magnetic yokes distalend portions of the lugs of the other magnetic yoke are arrangedalternately in the circumferential direction, and an outer size of thepair of magnetic yokes in the axial direction is larger than or equal toa length of the magnet in the axial direction.
 2. The torque detectiondevice according to claim 1, wherein each bent portion is bent in adirection in which the lugs extend.
 3. The torque detection deviceaccording to claim 1, wherein each bent portion is bent in a directionopposite to a direction in which the corresponding lugs extend.
 4. Anelectric power steering system, comprising: the torque detection deviceaccording to claim 1, which detects a steering torque applied to asteering wheel; an electric motor that generates driving force forassisting steering operation of the steering wheel; and a control unitthat controls the electric motor on the basis of the steering torquedetected by the torque detection device.